EXAMPLE OF A SIMPLE PATCH

Here's something Larry suggested: if a U
is the first active format
during a pack, (for example, pack"U3C8",@stuff
) then the
resulting string should be treated as UTF-8 encoded.

If you are working with a git clone of the Perl repository, you will
want to create a branch for your changes. This will make creating a
proper patch much simpler. See the perlgit for details on how to do
this.

Writing the patch

How do we prepare to fix this up? First we locate the code in question
- the pack happens at runtime, so it's going to be in one of the
pp files. Sure enough, pp_pack
is in pp.c. Since we're going
to be altering this file, let's copy it to pp.c~.

[Well, it was in pp.c when this tutorial was written. It has now
been split off with pp_unpack
to its own file, pp_pack.c]

Now let's look over pp_pack
: we take a pattern into pat
, and then
loop over the pattern, taking each format character in turn into
datum_type
. Then for each possible format character, we swallow up
the other arguments in the pattern (a field width, an asterisk, and so
on) and convert the next chunk input into the specified format, adding
it onto the output SV cat
.

How do we know if the U
is the first format in the pat
? Well, if
we have a pointer to the start of pat
then, if we see a U
we can
test whether we're still at the start of the string. So, here's where
pat
is set up:

STRLENfromlen;

registerchar *pat = SvPVx(*++MARK,fromlen);

registerchar *patend = pat + fromlen;

registerI32len;

I32datumtype;

SV *fromstr;

We'll have another string pointer in there:

STRLENfromlen;

registerchar *pat = SvPVx(*++MARK,fromlen);

registerchar *patend = pat + fromlen;

+ char *patcopy;

registerI32len;

I32datumtype;

SV *fromstr;

And just before we start the loop, we'll set patcopy
to be the start
of pat
:

items = SP - MARK;

MARK++;

sv_setpvn(cat, "", 0);

+ patcopy = pat;

while (pat < patend) {

Now if we see a U
which was at the start of the string, we turn on
the UTF8
flag for the output SV, cat
:

+ if (datumtype == 'U' && pat==patcopy+1)

+ SvUTF8_on(cat);

if (datumtype == '#') {

while (pat < patend && *pat != '\n')

pat++;

Remember that it has to be patcopy+1
because the first character of
the string is the U
which has been swallowed into datumtype!

Oops, we forgot one thing: what if there are spaces at the start of the
pattern? pack(" U*",@stuff)
will have U
as the first active
character, even though it's not the first thing in the pattern. In this
case, we have to advance patcopy
along with pat
when we see
spaces:

OK. That's the C part done. Now we must do two additional things before
this patch is ready to go: we've changed the behaviour of Perl, and so
we must document that change. We must also provide some more regression
tests to make sure our patch works and doesn't create a bug somewhere
else along the line.

Testing the patch

The regression tests for each operator live in t/op/, and so we make
a copy of t/op/pack.t to t/op/pack.t~. Now we can add our tests
to the end. First, we'll test that the U
does indeed create Unicode
strings.

t/op/pack.t has a sensible ok() function, but if it didn't we could use
the one from t/test.pl.

We now compile up Perl, and run it through the test suite. Our new
tests pass, hooray!

Documenting the patch

Finally, the documentation. The job is never done until the paperwork
is over, so let's describe the change we've just made. The relevant
place is pod/perlfunc.pod; again, we make a copy, and then we'll
insert this text in the description of pack: